I propose to estimate total body potassium by potassium-40 counting in a group of juvenile and adult subjects with diabetes mellitus. Measurements will be made only at times when the diabetic state is judged to be clinically stable, i.e., absence of recent episodes of acidosis, weight loss, or infection. From this measurement an estimate of lean body mass can be made, and data on diabetic subjects will be compared to those already obtained on a large series of normal subjects. The question at issue is whether the diabetic subject, ostensibly in good control, has a smaller body content of potassium (and hence presumably a smaller lean body mass) than the normal person of comparable age, height, and sex. Preliminary data suggest (but do not prove) that this is so. The answer to this question may have important implications for the welfare of the diabetic patient.